1) Field of the Invention
This invention relates to a device for electronic energy conversion between two three-phase systems without using voltage circuits or direct current-link (DC-link) power circuits.
2) Description of the Related Art
Electronic power converters (inverters) are used in many areas of industrial electronics. With such an electronic power converter, a controlled energy exchange can be achieved between two three-phase systems of different voltage and/or frequency. For certain applications in the areas of power-supply technology or drive engineering, a power flow with bidirectional capability is required, for example in the case of an asynchronous machine drive with regenerative braking, i.e. energy recovery in the supply network.
Conventional converters of this type are achieved by coupling, on the DC-link side, two three-phase pulse-controlled inverters, each consisting of six interruptible semiconductor valves, with a DC-link or an alternating current-link. Although this circuit topology makes the required energy conversion possible, an DC-link reactance element (DC-link capacitor or DC-link resistor) is required in addition to the semiconductor valves. This DC-link reactance element has, in principle, not insignificant current or voltage ratings, which contributes significantly to the unit volume or to the construction costs of the overall device.
Thus, to increase the specific power density and/or to reduce the construction costs of electronic converters for the exchange of energy between three-phase systems, an alternative converter configuration is also well-known, by means of which it is possible to avoid the described DC-link reactance elements and the energy conversion will occur quasi-directly (direct pulse converter). The conventional circuit topology is used here, with which each output phase of the converter can be switched by means of an electronic three-pole switch to any of the three phases of the supplying input voltage systems as desired. By appropriate pulse-width controlled clocking of the three-pole switch, any selectable (e.g. sinusoidal variable) voltage can be generated at the output within the control limits-mediated through the operating interval. The control of each individual three-pole switch occurs through the use of three appropriately interconnected and controlled single-pole electronic switches. Due to the resulting overall configuration (consisting of three phases for each three single-pole electronic switches), converters of this type are also designated as xe2x80x9cmatrix pulse convertersxe2x80x9d, because the required total of nine electronic switches are drawn in the circuit diagram mostly in the form of a 3xc3x973 matrix configuration. However, the disadvantage of this well-known configuration is that the nine electronic switches are bidirectionally loaded by the current and bipolar voltage, i.e. semiconductor elements are required for them that can be operated in all the four possible quadrants of the current/voltage characteristics. xe2x80x9cSymmetricallyxe2x80x9d interruptible electronic switches of this type, however, could not be produced so far in the relevant power range as single semiconductors for a variety of reasons related to semiconductor physics, but are usually materialized through a counter-series circuit of two power transistors, each with reverse diodes laid out in parallel. This requires a relatively large number of power transistors (a total of 18 MOSFETs or IGBTs), which is a major disadvantage of the direct-pulse inverters.
It is an object of this invention to provide a configuration, which enables electronic energy conversion between two three-phase systems thereby dispensing with an DC-link and has a reduced number of the required power transistors in comparison with the well-known direct-pulse converter configuration.
The device according to the present invention includes three half-bridge arms on its DC voltage side, specially formed bridge connections. Moreover, the DC-link capacitor is not provided. By an appropriate control of the input-side bridge connections it is ensured that the positive input voltage required for the proper functioning of the output-side conventional half-bridge connections is always available at the AC-side coupling point. According to the present device, it is possible to implement all the operating states of the well-known direct pulse converter. Moreover, the device possesses a smaller number of power transistors than the conventional direct-pulse converter.
These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed descriptions of the invention when read in conjunction with the accompanying drawings.